An important feature of solar cells is that the
voltage of the cell does not depend on its size,
and remains fairly constant with changing light
intensity. However, the current in a device is
almost directly proportional to light intensity and
size1. Figure 6 shows example I / V
curves for a single cell as a function of light
input:

A solar cell's power output can be characterized
by two numbers -- a maximum Open Circuit Voltage
(Voc,
measured at zero output current)
and a Short Circuit Current
(Isc,
measured at zero output voltage). Remember that
power can be computed via this equation:

P = I * V

So with one term at zero these conditions (V =
Voc
@ I = 0; V = 0 @ I = Isc)
also represent zero power. As you might then
expect, a combination of less than maximum
current
and voltage can be found that maximizes the power
produced. This condition is called, not
surprisingly, the "maximum power point". BEAMsolar
engine designs attempt to stay at (or near)
this point. The tricky part is building a design
that can find the maximum power point regardless of
lighting conditions2.

Note that single junction silicon solar cells
produce approximately 0.5 - 0.6 Voc,
so they are usually connected together in series
to provide larger voltages. In some cases (like the
Panasonic Sunceram cells), multiple cells are built
onto a single substrate in order to yield the
convenience of higher output voltage from a single
package.

Some more subtle properties of solar cells also
need to be accounted for in their use. In
particular, when connecting solar cells in series,
care needs to be taken to give all cells roughly
equal access to light -- the weakest solar cell in
series (or one that is shaded) will determine the
total current. Normally this is not an issue in
BEAMbots,
and will only rear its ugly head if you spread
solar cells around on the surface of your 'bot. In
a pinch, reverse Schottky diodes can be wired
across each cell to automatically bypass any cell
that may get shaded.3

Similar issues can occur when wiring solar cells
in parallel. In that case a shaded cell can act as
a short circuit to the output of its more active
neighbors. Here, a germanium diode in series with
each separate cell can be used to mitigate
problems, if 'bot geometry can't be adjusted to
avoid them (although at the cost of a few tenths of
a volt).

A wide variety of cells are available for your
purchase. Rather than repeating a large amount of
solar cell selection and comparison information,
the reader is directed to consult the solar
cell section of the BEAM
Reference Library's BEAM
Pieces collection for help in deciding which
cells to buy.

For more
information

For more detailed information on the
physics of solar cells, I'd recommend
these sites (the sources of much of the
information in this report):